Found the mistake myself!
You have to spell the key with a capital
AlreadyLoggedIn instead of alreadyLoggedIn
Where do I need to put the org.jboss.seam.security.AlreadyLoggedIn message?
This is from the source code, you should add the key in your messages_en.properties file.
public void addAlreadyLoggedInMessage()
"You are already logged in, please log out first if you wish to log in again"